The Bluefin 21, the Artemis autonomous underwater vehicle (AUV), is hoisted back on board the Australian navy's Ocean Shield in the southern Indian Ocean. (U.S. Navy)

PERTH, Australia – Planes, ships and two submersible sound locators were deployed Saturday to again scour a remote patch of the Indian Ocean in the increasingly urgent hunt for the Malaysia Airlines jet that vanished four weeks ago.

A multinational team is desperately trying to find debris floating in the water or faint sound signals from the plane's flight recorders that could lead them to the aircraft and help unravel the mystery of its fate.

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Beacons in the black boxes emit "pings" so they can be more easily found, but the batteries only last about a month. And officials say that the more time that passes before any floating wreckage is found, the harder it will be to find the plane itself.

So far, there's been no sign of the Boeing 777.

The recorders could help investigators determine why Flight 370, which disappeared March 8 while en route from Kuala Lumpur to Beijing with 239 people aboard, veered so far off-course.

Two ships with sophisticated equipment that can hear the recorders' pings were deployed for the first time on Friday along a 150-mile route investigators hope may be close to the spot where they believe the plane went down.

Those ships, the Australian navy's Ocean Shield and the British HMS Echo, were returning to the search area on Saturday, along with up to 13 military and civilian planes and nine other ships, the agency coordinating the search said.

Weather conditions in the area, which have regularly hampered crews trying to spot debris, were fair with some rain expected, the Joint Agency Coordination Centre said.

Because the U.S. Navy's pinger locator can pick up signals to a depth of 20,000 feet, it should be able to hear the plane's data recorders even if they are in the deepest part of the search zone -- about 19,000 feet. But that's only if the locator gets within range of the black boxes -- a tough task, given the size of the search area and the fact that the pinger locator must be dragged slowly through the water at just 1 to 6 mph.

Officials said there was no specific information that led to the underwater devices being used for the first time on Friday, but that they were brought into the effort because there was nothing to lose.

Australian Air Chief Marshal Angus Houston, head of the joint agency coordinating the operation, acknowledged the search area was essentially a best guess, and noted the time when the plane's locator beacons would shut down was "getting pretty close."

Finding floating wreckage is key to narrowing the search area, as officials can then use data on currents to try to backtrack to where the plane hit the water, and where the flight recorders may be.

The overall search area is a 84,000-square-mile zone in the southern Indian Ocean, about 1,100 miles northwest of the western Australian city of Perth.

The search area has shifted each day as investigators continue to analyze what little radar and satellite data is available while factoring in where any debris may have drifted.

Australia is coordinating the ocean search, and the investigation into the plane's disappearance is Malaysia's responsibility. Australia, the U.S., Britain and China have all agreed to be "accredited representatives" of the investigation.

Meanwhile, the Air Line Pilots Association, a union that represents 30,000 pilots in North America, said in a statement that the Malaysia Airlines tragedy should lead to higher standards of plane tracking technology being adopted by the airline industry.